"They didn't win the last one and it is extremely rare for governing parties to improve their share of the vote, even when their opponents are as weakly led as Labour and the Liberal Democrats are now," he wrote.

Portillo's intervention is the first since former chancellor Nigel Lawson said he would vote against staying in the EU regardless of the deal Cameron hammered out on the continent.

That move was a major shot in the arm for eurosceptics, who are currently riding high as the Conservative party reacts to the threat to its right flank from Ukip.

Last night, the Conservatives returned the whip to backbench MP Nadine Dorries, partly out of the fear that she could be tempted into the arms of Nigel Farage's party.

Writing in the Times, Portillo said: "The default position of the political class is defeatism: the belief that Britain could not survive outside the union; and the political class assumes that the public shares its defeatism.

"The UK is unhappy in the EU. We do not share its vision, partly because we are not visionary by temperament.

"We are not so easily convinced that the EU is a necessary response to the horrors of the Second World War because our experience of that war was different. We did not endure revolution, dictatorship or invasion.

"Other countries may look to institutions at the European level because they doubt the durability of national institutions that perished in that conflict. We do not, because ours survived."

Cameron plans to hold an in/out referendum in the next parliament after securing a new deal for Britain but European partners will be resistant to allowing states to formulate their own membership criteria.